Study: Males With Young Stay Loyal

So a guy's sitting at a bar and a sweet young thing starts flirting. He's married, and he knows he should watch his step, but even though the conscience says no, the flesh is more than willing.

Would he be any less inclined to take that dangerous step toward promiscuity if, in addition to being married, he's also a dad?

If he's a dad, he'll almost certainly turn away and head back for the home and hearth, at least if he's a marmoset monkey. And quite likely if he's a human.

Researchers at the University of Wisconsin's National Primate Research Center in Madison were themselves quite surprised to learn that being a parent makes a huge difference in the sexual proclivities of marmosets. These squirrel-sized monkeys have a social structure that is very similar to ours, more so than any other primates, so they are the object of much fascination among scientists.

Mysterious Indifference

It's not always easy to study the sex lives of humans because it's pretty hard to maintain all the necessary controls, says Toni Ziegler, an endocrinologist who studies the effect of the social environment on hormones and behavior. So Ziegler, a senior scientist at the Wisconsin center, teamed up with Charles Snowdon, chairman of the department of psychology, to see if male marmosets could shed a little light on an important subject.

They wanted to know what happens to hormone production when a male marmoset, and by extension possibly a human, gets turned on by a seductive female. Does the male's body begin immediately to produce the hormones associated with sex, like testosterone, when given some kind of stimulus, like the smell of a very friendly female?

The obvious answer would be yes, because marmosets have a well-known affection for affection.

"We had just started testing when I said 'wait a minute, these guys aren't showing the normal physiological response,' " Ziegler says.

Thirty marmosets were participating in the study and two-thirds did just what they were expected to do. Testosterone spiked within minutes, showing they were more than ready to monkey around.

But about a third of them showed no real sign of interest. The increase in testosterone, for example, was virtually zilch.

"It was terribly exciting," says Ziegler, because nothing excites a good scientist more than an unexpected result that flies in the face of conventional wisdom. What could be wrong with all these monkeys that showed no interest in sex, even when given appropriate stimulants?

Hormone 'Spigot' Switches Off

It took a little research to find out. All of the males who turned away from the promise of illicit seduction were dads. Some were new dads, some were pretty new dads, some were old dads, but all showed responses that ranged from zero to barely alive.

The reason, apparently, stems from their unusual social structure. Unlike other primates, marmosets are monogamous with "committed couples" giving birth to two sets of twins every year, Snowdon says. Every member of the family is expect to pitch in and help with all the household chores, including raising the young. That includes dad who must share in the considerable work associated with raising a whole bunch of kids.

That social structure is believed to "cause a muted physiological response in fathers" to any female marmoset, except for the love of his life, says Ziegler, lead author of a report on the research in a recent issue of the journal Hormones and Behavior. So in this case the physiology of the male suppresses the production of the hormones that could otherwise cause him a great deal of grief.

So for old dad, it's not a strength of character issue. The spigot, so to speak, has been turned off.

Does that make marmosets unique? Other studies have shown that various mammals, like pigs and bulls, respond as expected to a sexual stimulus. Within 30 minutes, their systems are charged and they've got all the hormones they need to take care of business. The same effect, by the way, has been shown among humans who are exposed to erotic materials.

But until now, there has been no reason to think that simply being a parent has any impact on the physiological processes needed to fire up the male. For marmosets, at least, it makes a difference indeed.

It will take a lot more research to see if anything similar to that happens to human males. Given the similarities between the social structures of humans and marmosets, it would not be surprising if being a dad at least makes some difference.

Snowdon says that would be good news.

"I'm tired of all men getting a bad rap for being supposedly promiscuous and irresponsible," he says. "I'm happy that we've finally found a species where, as parents, both females and males do the heavy lifting."

Incidentally, he's not a dad.

Lee Dye's column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.